Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 222329

Area Forecast Discussion
National Weather Service New York NY
729 PM EDT Sat Jul 22 2017

A frontal boundary remains to our south tonight as a wave of
low pressure tracks along it, this front then sags farther to
the south on Sunday after the low passes. Another low then
moves along the front, tracking off the mid-Atlantic coast by
Monday morning. Still another area of low pressure that moves
through the region Monday and Monday night, exits to the east
Tuesday. High pressure builds over the northeast late Tuesday
through Wednesday, then drifts off the coast Wednesday night.
Another wave of low pressure follows for Wednesday night into
Friday, with a cold front moving through Friday. High pressure
returns Saturday.


The peak heating passed and heat indices were in the upper 80s
to around 90 allowed the heat advisory to expire.

With the main area of instability to the south of the region,
and southwest, across central and southern New Jersey, to south
of Long Island have updated the probabilities and mention of
thunder through tonight, with chance and isolated wording for
thunderstorms. Also, with the main threat of stronger storms to
the south removed any enhanced wording. The best storm was
tracking across southeastern Pennsylvania and moving into
southern New Jersey. The HRRR was also indication this downward
trend for the stronger convection to remain to the south.

Once shortwave exits, the overnight hours should be mainly dry,
except for possibly some lingering showers mainly over Long

Lows tonight should be around 5 degrees above normal.

There is a moderate risk for the development of rip currents at
ocean beaches through this evening.


The region should be under locally zonal flow Sunday. With no
shortwaves of note forecast to move over the region in this
flow, it should be mainly dry. However, this is quite a change
from the previous forecast. So to trend things, went with
slight chance pops over western zones and mainly dry over
eastern zones through late afternoon, then increased pops to
chance over far western zones and slight chance throughout
elsewhere (reflecting climatological trends for convection).

A sharpening 700-500 hPa trough/developing closed low moving
into the Great Lakes Sunday night, along with 850 hPa
frontogenesis, will produce an increasing threat of showers,
with a slight chance of thunderstorms. The pops increase from W
to E, to likely throughout by after midnight.

Onshore low-level flow should result in a mostly cloudy sky on
Sunday, limiting highs to near to slightly below normal levels
(mainly around 80-mid 80s). Lows Sunday night should be slightly
above normal due to cloud cover.

There is a moderate risk for the development of rip currents at
ocean beaches Sunday.


An upper level shortwave and accompanying wave of surface low
pressure will be exiting to the east Monday night into early
Tuesday. Showers and isolated thunderstorms will be on-going at the
beginning of the extended forecast period. Instability and CAPE will
be marginal and increase somewhat during Monday, however may still
not be sufficient to support widespread convection, so will keep
isolated wording.

The upper westerly flow will be progressive through the period. A
weak ridge builds Monday night into Tuesday, and Wednesday. Then
another shortwave, with an embedded closed low, over the Canadian
west coast Monday, opens and digs a more significant trough into
the eastern states Wednesday night into Thursday with a high
amplitude trough, of 1 to 2 standard deviations above normal,
digging all the way to northern Florida by Friday. The flow looked
to be still progressive with a surface low moving through
Thursday and Friday. However, there are hints that the upper low
will close off again Friday into Saturday and remain along the
northern coast into next weekend. At this time will keep with
persistence and the more progressive flow and keep Friday night
into Saturday dry as upper ridging builds to the north.

Temperatures through the extended period will be near to slightly
below seasonal normals.


A weak cold front will remain nearly stationary just south of
the area through tonight. Low pressure will pass just south
along this front tonight. Expect mostly VFR conditions through
the TAF period, outside of any showers and thunderstorms that
are forecast.

Showers and isolated thunderstorms over eastern Pennsylvania
will continue to be watched as they move eastward tonight. Best
timing for any thunder is 01Z-04Z for the NYC terminals.
Chances are lower, as well as confidence, for eastern terminals
such as KISP and KBDR where only VCTS is mentioned from 00Z-
10Z, and there is no mention of thunder for KGON, however an
isolated thunderstorm is possible for the same time period. Then
rain is expected through 09-10Z, with drier conditions expected
on Sunday.

Any showers and thunderstorms have the potential to lower conditions
to MVFR or lower.

Winds will shift from the south to the northeast tonight, then shift
to the southeast on Sunday for most terminals, with speeds generally
10 kt or less.

.Sunday night-Monday night...Episodes of SHRA/TSRA with MVFR or
lower conditions possible.
.Tuesday...An AM shower possible at KGON, otherwise VFR.
.Tuesday night-Wednesday night...VFR.
.Thursday...Sub-VFR possible in SHRA/TSRA


A light to moderate pressure gradient over the waters around
Long Island through Sunday night, will limits sustained winds
to 15 KT or less, ocean seas to 4 ft or less, and seas/waves on
the non- ocean waters to 1 ft or less through then.

Winds, gusts, and seas will remain below small craft levels Monday
through Thursday. However, ocean seas may approach 5 feet Wednesday
night into Thursday as an increasing southerly flow develops ahead
of a cold front. Waves of low pressure passing through the waters
will bring a chance of thunderstorms Monday and again Thursday into
Thursday night.


Currently expecting around 1/4-1/2 inch of rainfall across
southern portions of the CWA and less than 1/4 of an inch of
rainfall elsewhere through Sunday morning. Locally heavy
rainfall is possible with any stronger convection tonight,
mainly over southern zones. If the locally heavy rainfall
occurs, it could result in at most minor flooding of urban and
poor drainage areas.

An additional 1/2 to 1 inch of rainfall is likely from Sunday
night into Monday night, with locally higher amounts possible.
There is a small chance for minor flooding or urban and poor
drainage areas, if any experience locally heavy rainfall.

More rain is possible late Wednesday night through Friday.
Significant hydrologic impacts are not expected at this time.


With the approach of the new moon (Sun), tides will run high
this weekend into early next week. Positive tidal departures of
1/2 to 1 ft will be needed for minor flooding during the night
time high tides during this time.

Minor flooding is possible starting with the high tide cycle
this evening, mainly in the western south shore bays of Long
Island. A bit more widespread minor flooding is possible Sunday
night with an E/NE flow expected. The threat for minor flooding
could continue into Monday, with E/NE flow progged to continue.




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